My toilet started not quite flushing properly about 50% of the time.
I looked inside the tank & noticed:
- sometimes it fills to the max line, and then when I flush, it flushes properly
- it sometimes fills only half way, and then when I flush, it only flushes a bit.. And I have to throw some water into the bowl & it will finish flushing

So is there something I can do to make sure that the tank gets filled up to the max line every time?

Thank you!

I noticed something else - sometimes when the tank is filled to the max, it will slowly drain down which then causes the toilet not to flush

Which ballcock is yours? (see image )
sometimes when the tank is filled to the max, it will slowly drain down which then causes the toilet not to flush Run your fingers along the edge of the flapper. If it's wavy or rough replace the flapper. Let me know about your ballcock. Regards, nTom

Tom is taking you down the right road. Your fill valve is shot. You can repair them but for under $10 you can replace them with a worry free Fluidmaster 400A, left picture

Hi Tom - I have the one on the left. Thanks!

This ballcock has a problem of the intake opening of the seal swelling shut. Simple fix! Go to HD or Lowe's and ask for a FluidMaster Replacementg Seal# 242.
Shut the water off to the toilet and remove the lid. Take and twist the top off the ballcock,(see image) and replace the seal. Reassemble and give it a flush. Don't forget to position the small tube back on the overflow tube. Good luck, Tom